1. Field of the Invention
The present invention relates to a light intensity distribution converting device which converts the intensity distribution of incident light to output the converted light, and to an optical data storage apparatus using the same.
2. Description of the Related Art
An optical system for an optical data storage apparatus comprises a laser source, a collimator lens, and an objective lens. In such a system, the diameter of a beam spot is required to be small to enhance the density of an optical disc apparatus, and an increase in the quantity of light is required to improve the transmission speed.
To minimize the spot diameter in conventional optical systems, attempts have been directed to an increase in the numerical aperture NA of an objective lens or reduction of the wavelength of the laser. However, an increase in the NA of an objective lens tends to enhance aberrations. Shortening of the laser wavelength relies upon an improvement of the laser source proper and cannot be realized by improving the optical disc apparatus. Also, the quantity of light can be increased only by a realization of a higher power laser.
In optical theory, it is known that when condensing a pencil of light, a pencil of light having a uniform light intensity distribution can form a beam spot diameter smaller, and close to the diffraction limit, than a pencil of light having a Gaussian light intensity distribution. This effect is equivalent to a provision of a spot diameter, using a laser whose wavelength is shorter than that of the laser being used by several tens of nm, for the objective lens of the same NA.
In general, since the light intensity on a wave front of the pencil of light has a Gaussian distribution, the aperture of an objective lens upon which the light is made incident is restricted. Therefore, efforts have been made to minimize the beam spot diameter by making only the light in the vicinity of an optical axis, whose intensity distribution can be considered approximately uniform, incident upon the objective lens. In the attempts, naturally, no light outside the aperture diameter is used, and therefore, the utilization efficiency of light of the light source is reduced. If the aperture diameter to be used is expanded to cover light whose intensity distribution cannot be considered uniform in order to increase the light utilization efficiency, the beam spot diameter will be then larger than that for the uniform light intensity distribution. As can be understood from the foregoing, the uniformity of the light intensity is not compatible with the reduction of loss of the quantity of light.
A light intensity distribution converting device which converts collimated light having a Gaussian intensity distribution into collimated light having a uniform light intensity distribution has been proposed in Japanese Patent Application No. 10-57003 filed prior to the present application. This light intensity distribution converting device can be disposed between a collimator lens and an objective lens in an optical disc apparatus which is comprised of a laser source, the collimator lens, and the objective lens. The objective lens receives parallel light beams having a uniform light intensity distribution, so that a small diameter of a beam spot can be obtained. However, this light intensity distribution converting device is an additional element in the optical disc apparatus.
Japanese Unexamined Patent Publication No. 63-188115 discloses a beam forming optical apparatus in which light having a Gaussian light intensity distribution can be converted to light having a uniform light intensity distribution by two lenses. In this prior art, the two lenses are constructed so as not to satisfy the sine condition. However, this requires a small machining tolerance. Therefore, to reduce the tolerance, one of the two lenses is designed to dissatisfy the sine condition and the other is designed to satisfy the sine condition. In this solution in which the light intensity distribution is converted using spherical aberrations, a wave front aberration is inevitably produced, and therefore, the optical device cannot be used as a small optical element for an optical disc apparatus or the like.
An object of the present invention is to provide a light intensity distribution converting device which can convert a given light intensity distribution to a desired one, so that wave front aberrations and the beam spot diameter can be reduced, and also to provide an optical data storage apparatus using the light intensity distribution converting device.
According to the present invention, there is provided a light intensity distribution converting device comprising a body having a central axis, a first curved surface transversely extending with respect to the central axis, a second curved surface transversely extending with respect to the central axis, and an outer circumferential surface extending between said first and second curved surfaces, one of the first and second curved surfaces having a concave surface configuration at approximately a center portion thereof, the other having a convex surface configuration at or near a center portion thereof, at least one of incident light and emerging light is diverging or converging light and the first and second curved surfaces being formed such that the light intensity distribution of incident light, due to refractions at the first and second curved surfaces is different from that of emerging light while the light passes from the first curved surface to the second curved surface.
Using this light intensity distribution converting device, emerging light having a uniform intensity distribution can be obtained from incident light having a Gaussian intensity distribution. In an optical data storage apparatus according to the present invention, the light intensity distribution converting device is used as a collimator lens and/or an objective lens. For example, if the light intensity distribution converting device is used as a collimator lens for an optical disc apparatus, the objective lens of the device receives light having a uniform intensity distribution, so that a smaller beam spot diameter can be obtained. If the light intensity distribution converting device is used as an objective lens for an optical storage apparatus, the objective lens has functions to convert the light intensity distribution and to condense the light.